org-persist.el: Do not use hook depth unnecessarily

* lisp/org-persist.el: Do not use hook depth to define the order of
`org-persist-gc' and `org-persist-write-all'.  Rely on default
behaviour of `add-hook' instead.
(org-persist-register): Use default hook depth.

lisp/org-persist.el

@@ -123,7 +123,7 @@ dependency means that data shared between variables will be preserved
                          (cons var (plist-get inherited-index :variable)))))))
   (org-persist--get-index var buffer)
   (when buffer
-    (add-hook 'kill-buffer-hook #'org-persist-write-all-buffer 1000 'local)))
+    (add-hook 'kill-buffer-hook #'org-persist-write-all-buffer 'local)))
 
 (defun org-persist-unregister (var &optional buffer)
   "Unregister VAR in BUFFER to be persistent.
@@ -285,8 +285,10 @@ When BUFFER is `all', unregister VAR in all buffers."
   (if (not (file-writable-p dir))
       (message "Missing write access rights to org-persist-directory: %S"
                org-persist-directory)
-    (add-hook 'kill-emacs-hook #'org-persist-gc)
-    (add-hook 'kill-emacs-hook #'org-persist-write-all 100)))
+    (add-hook 'kill-emacs-hook #'org-persist-write-all)
+    ;; `org-persist-gc' should run before `org-persist-write-all'.  So we are adding the
+    ;; hook after `org-persist-write-all'.
+    (add-hook 'kill-emacs-hook #'org-persist-gc)))
 
 (add-hook 'after-init-hook #'org-persist-read-all)